A superconducting accelerator cavity accelerates charged particles that pass through the interior thereof. This superconducting accelerator cavity is formed by connecting beam pipes to ends of a cavity main body, which is a main body of the cavity, in which a plurality of cells with circular tube shapes having swollen center portions are combined. The cavity main body and the beam pipes are made of, for example, niobium, which is a superconducting material.
In order to maintain a superconducting state, at least the cavity main body needs to be kept in an extremely low-temperature state. Because of this, the area surrounding the cavity main body is generally surrounded by a titanium or stainless steel jacket, and the cavity main body is cooled to the extremely low-temperature state by accommodating, for example, liquid helium inside the jacket.
At this time, it is important to maintain airtightness at joints between the jacket and the superconducting accelerator cavity. Although the joints are conventionally joined by interposing gaskets therebetween or are joined by using brazing filler metals, this has not been enough to achieve sufficient airtightness.
As disclosed in Patent Literature 1, in order to achieve sufficient airtightness, it has been proposed to provide a niobium ring with protrusions, which has protruding portions over the entire circumference of an outer circumferential portion thereof, to join the titanium jacket to tips of the protruding portions by welding, followed by joining of the cavity main body and the beam pipes to both ends of the ring with the protrusions by welding.